This invention relates to a method of improving the stability of aromatic polycarbodiimides. More particularly, this invention relates to a method of improving the stability of aromatic polycarbodiimides using catalyst poisons.
Aromatic polycarbodiimides (hereinafter referred to as xe2x80x9car-pCDIxe2x80x9d) are used as crosslinkers in a variety of applications, including coatings, adhesives, caulks, mastics, and the like. Generally, ar-pCDI are made by heating aromatic isocyanates in the presence of phosphorous oxides, usually phospholene oxides such as 3-methyl-1-phenyl-2-phospholene-1-oxide (hereinafter referred to as xe2x80x9cMPPOxe2x80x9d). The phosphorus oxides catalyze the reaction between 2xe2x80x94NCO groups to make a carbodiimide (hereinafter referred to as xe2x80x9cCDIxe2x80x9d) and CO2:
2R-NCOxe2x86x92Rxe2x80x94Nxe2x95x90Cxe2x95x90Nxe2x80x94R+CO2
The catalyst reacts with xe2x80x94NCO to form a phosphinimide and carbon dioxide:
Rxe2x80x2NCO+R3Pxe2x95x90Oxe2x86x92R3Pxe2x95x90NRxe2x80x2+CO2
that then reacts with another Rxe2x80x2NCO to regenerate the catalyst and make CDI:
R3Pxe2x95x90NRxe2x80x2+Rxe2x80x2NCOxe2x86x92Rxe2x80x2Nxe2x95x90Cxe2x95x90NRxe2x80x2+R3Pxe2x95x90O.
After they are made, the ar-pCDI molecules begin to slowly build viscosity until they gel. While not wishing to be bound by theory, it is believed that the residual phosphorous oxide present in ar-pCDI solutions catalyze the reaction between the ar-pCDI and any compounds containing active hydrogens such as water, alcohols, amines, and other materials bearing active hydrogens that may contaminate the ar-pCDI solutions:
Nxe2x95x90Cxe2x95x90N+ROHxe2x86x92Nxe2x95x90C(OR)NH
Over time, the accumulation of Nxe2x95x90C(OR)NH causes the ar-pCDI to build viscosity and eventually gel.
This instability limits the usefulness of ar-pCDI because the shelf life is too short, i.e., less than six months, to be commercially viable. Aliphatic pCDI do not suffer the same stability problems as ar-pCDI, but, inter alia, are too reactive to make good crosslinkers for water-borne coatings when compared with ar-pCDI. Thus, ar-pCDI would be preferred if the shelf-life stability problems could be solved.
I have discovered that the stability or gel time of ar-pCDI is dependent on the level of phosphorous oxide used to make it and that remains in the ar-pCDI after manufacture; the more phosphorous oxide, the shorter the gel time. I have further discovered a way to deactivate these catalysts. Certain chemicals, hereinafter referred to as xe2x80x9ccatalyst poisons,xe2x80x9d may be added to the ar-pCDI after it is made that will react with the catalyst to deactivate it, thus greatly extending the storage stability or shelf-life of the ar-pCDI.
These catalyst poisons are not novel. For example in U.S. Pat. No. 5,202,358, U.S. Pat. No. 4,014,935, and U.S. Pat. No. 4,614,785, organic isocyanates containing carbodiimides and/or uretone imine groups are prepared by the partial polymerization of the xe2x80x94NCO groups using phosphorous oxide .catalysts. When only partial conversion takes place, the carbodiimides react with further isocyanate groups to give uretone imine groups. To ensure only a partial polymerization of the xe2x80x94NCO groups, U.S. Pat. No. 5,202,358 discloses the addition of silylated acid compounds of the formula Xxe2x80x94[Si(CH3)3]n to the reaction to terminate the formation of carbodiimide groups. To ensure only a partial polymerization of the xe2x80x94NCO groups, U.S. Pat. No. 4,014,935 discloses that the phosphorous oxide catalyst is absorbed onto a substrate or deactivated by halides of hydrogen, tin, or phosphorus, or the oxyhalide of phosphorus or sulfur. To ensure only a partial polymerization of the xe2x80x94NCO groups, U.S. Pat. No. 4,614,785 discloses that the phosphorus oxide catalyst is deactivated with sulfonyl isocyanates.
In each of the above-referenced patents, the starting material is an isocyanate catalyzed by a phosphorous oxide. Only a partial reaction is desired so that the final product is a mixture. However, the presence of the residual phosphorous oxide leads to an undesired, complete reaction. The deactivation of the catalyst, either by adsorption onto a substrate or by the addition of a catalyst poison, prevents complete conversion to the pCDI. The present invention utilizes the deactivation steps but their purpose is to prevent the residual phosphorous oxide in the completed reaction from gelling the system rather from preventing the completed reaction.
The invention is directed to a method of improving the stability of ar-pCDI formed using a phosphorous oxide catalyst including the step of deactivating the catalyst.
Preparation of ar-pCDI using phosphorous oxide catalysts may be by conventional means such as those methods disclosed in U.S. Pat. No. 2,853,473, U.S. Pat. No. 2,941,966, U.S. Pat. No. 2,941,983, U.S. Pat. No. 4,487,964 and U.S. Pat. No. 5,574,083. On completion of the preparation of ar-pCDI, the phosphorous oxide catalysts is deactivated by addition to the ar-pCDI of at least one catalyst poison.
Deactivation may be carried out by addition to the ar-pCDI of at least one catalyst poison. The catalyst poisons useful in the method of invention include sulfonyl isocyanates and silylated acids of the formula Xxe2x80x94[Si(CH3)3]n where X represents the neutral acid residue obtained by removal of the acidic hydrogen atoms from an n-basic acid having a pKa value of at most 3, other than a hydrohalic acid, and n is an integer of 1 to 3.
Examples of suitable sulfonyl isocyanates include any inorganic or organic compounds which contain at least one structural unit corresponding to the following formula xe2x80x94SO2xe2x80x94NCO. Organic sulfonyl isocyanates are preferably used, while those containing aromatically-bound isocyanatosulfonyl residues are particularly preferred. Processes for producing organic sulfonyl isocyanates of the type suitable for use in accordance with the invention and also their chemical behavior are comprehensively described by H. Ulrich in Chem. Rev. 65, pages 369-376, 1965. In addition, the production of aryl sulfonyl isocyanates is described in U.S. Pat. No. 2,666,787 and U.S. Pat. No. 3,484,466. According to the invention, it is possible to use aliphatic, cycloaliphatic and also aromatic mono- or polysulfonyl isocyanates, of which the following are mentioned by way of example: methyl sulfonyl isocyanate, butyl sulfonyl isocyanate, cyclohexyl sulfonyl isocyanate, chlorosulfone isocyanate, perfluorooctyl sulfonyl isocyanate, phenyl sulfonyl isocyanate, p-toluene sulfonyl isocyanate, benzyl sulfonyl isocyanate, p-chlorophenyl sulfonyl isocyanate, m-nitrophenylsulfonyl isocyanate, 2,5-dimethyl phenyl sulfonyl isocyanate, p-fluorophenyl sulfonyl isocyanate, 2,5-dichlorophenyl sulfonyl isocyanate, 3,4-dichlorophenyl sulfonyl isocyanate, p-bromophenyl sulfonyl isocyanate, p-methoxyphenyl sulfonyl isocyanate, p-nitrophenyl sulfonyl isocyanate and o-nitrophenyl sulfonyl isocyanate; m-phenylene disulfonyl diisocyanate, p-phenylene disulfonyl diisocyanate, 4-methyl-m-phenylene disulfonyl diisocyanate, 2-chloro-p-phenylene disulfonyl diisocyanate, 5-chloro-m-phenylene disulfonyl diisocyanate, 1,5-naphthylene disulfonyl diisocyanate, 3-nitro-p-phenylene disulfonyl diisocyanate, 4-methoxy-m-phenylene disulfonyl diisocyanate, 2,5-furandiyl-bis-(methylene-sulfonyl)-diisocyanate, 4,4xe2x80x2-bis-phenylene disulfonyl diisocyanate, 2,2xe2x80x2-dichloro-4,4xe2x80x2-biphenylylene-disulfonyl diisocyanate, 3,3xe2x80x2-dimethoxy-4,4xe2x80x2-biphenylylene-disulfonyl diisocyanate, (methylene-di-p-phenylene)-disulfonyl diisocyanate, (methylene-di-3,3xe2x80x2-dimethoxy-p-phenylene)-disulfonyl diisocyanate, (methylene-di-3,3xe2x80x2-dimethyl-p-phenylene)-disulfonyl diisocyanate and 2-methyl-p-phenylene disulfonyl diisocyanate; also sulfonyl isocyanates containing free NCO-groups such as m-isocyanatophenyl sulfonyl isocyanate, p-isocyanatophenyl sulfonyl isocyanate, 3-isocyanato-p-tolyl sulfonyl isocyanate, 5-isocyanato-o-tolyl sulfonyl isocyanate, 3-isocyanato-4-methoxyphenyl sulfonyl isocyanate, 4-isocyanato-3-chlorophenyl sulfonyl isocyanate, 4xe2x80x2-isocyanato-4-biphenylyl sulfonyl isocyanate, 4xe2x80x2-isocyanato-2,2xe2x80x2-dichloro-4-biphenylyl sulfonyl isocyanate, 4-isocyanato-3,3xe2x80x2-dimethoxy-4-biphenylyl sulfonyl isocyanate, xcex1-(p-isocyanatophenyl)-p-tolyl sulfonyl isocyanate, xcex1-(4-isocyanato-3-methoxyphenyl)-2-methoxy-p-tolyl sulfonyl isocyanate, xcex1-(4-isocyanato-m-tolyl)-2,4-xylyl sulfonyl isocyanate and 5-isocyanato-1-naphthyl sulfonyl isocyanate; or containing free isothiocyanate groups such as p-isothiocyanatophenyl sulfonyl isocyanate, m-isothiocyanatophenyl sulfonyl isocyanate, 3-isothiocyanate-4-methoxy phenyl sulfonyl isocyanate and 4-isothiocyanato-3-methyl phenyl sulfonyl isocyanate.
It is preferred to use sulfonyl isocyanates wherein the xe2x80x94SO2xe2x80x94NCO group is directly attached to an aromatic radical. Phenyl sulfonyl isocyanate, p-chlorophenyl sulfonyl isocyanate and p-toluene sulfonyl isocyanate (tosyl isocyanate) are particularly preferred. In addition to the organic sulfonyl isocyanates mentioned by way of example, it is also possible in accordance with the invention to use inorganic sulfonyl isocyanates such as chlorosulfonyl isocyanate or sulfonyl diisocyanate. Oxy-sulfonyl isocyanates such as trimethyl silyloxy-sulfonyl isocyanate are also suitable.
Examples of suitable silylated acids of the formula Xxe2x80x94[Si(CH3)3]n include silylated sulfonic acids, such as trifluoromethanesulfonic acid trimethyl silyl ester and methanesulfonic acid trimethylsilyl ester; and silylated esters of acids of phosphorous, such as phosphoric acid tris(trimethylsilyl ester) and phosphoric acid diethyl ester trimethylsilyl ester.
The catalyst poison may be added to the ar-pCDI at a molar ratio of 0.1:1 to 30:1 (catalyst poison:phosphorus oxide catalyst), preferably, 0.3:1 to 5:1. The catalyst poison may be added neat or as a solution in a suitable solvent.
Compositions containing ar-pCDI made by the method of the present invention are more stable to storage at room temperature, especially up to 6 months, and are more thermally stable at elevated temperatures, for example of up to 80-120xc2x0 C. than those compositions containing ar-pCDI made by conventional methods.
Some embodiments of the present invention will now be described in detail in the following Examples.